SURFICIAL GEOLOGIC MAP OF THE LEVAN AND FAYETTE SEGMENTS OF THE WASATCH FAULT ZONE, CENTRAL UTAH

New surficial geologic mapping, supplemented with field measurement and analysis of 52 fault-scarp profiles, characterizes the Quaternary geology and spatial and temporal patterns of surface faulting along the Levan and Fayette segments of the Wasatch fault zone (WFZ). Our mapping of these two southernmost segments of the WFZ completes 1:50,000-scale surficial geologic mapping of the entire length of the WFZ affected by Holocene surface faulting.

Stratigraphic data and numerical ages indicate the most recent surface-faulting earthquake on the Levan segment occurred shortly after 1000 ± 200 cal yr B.P. Numerical ages roughly constrain the timing of the penultimate surface-faulting earthquake on the Levan segment to sometime prior to 2800–4300 cal yr B.P., and perhaps prior to 6000–10,600 cal yr B.P. On the Fayette segment, cross-cutting geologic relations and empirical analysis of scarp-profile data indicate the timing of most recent surface faulting is different for the three strands of the segment: early or middle Pleistocene(?) for the northern (N) strand, latest Pleistocene for the southeastern (SE) strand, and Holocene for the southwestern (SW) strand. The timing of earlier surface-faulting earthquakes on the Fayette segment is unknown.

Our preferred maximum Holocene vertical slip rate for the Levan segment is 0.3 ± 0.1 mm/yr. Estimated middle to late Quaternary vertical slip rates are 0.02–0.05 mm/yr for the Levan segment and 0.01–0.03 mm/yr for the Fayette segment; a locally higher rate (0.06–0.1 mm/yr) at the north end of the SW strand of the Fayette segment may result from spillover of Levan-segment ruptures onto the Fayette segment, or additive slip from separate SW- and SE-strand Fayette-segment ruptures that overlap on this part of the fault, or some combination of these two scenarios. Additionally, the higher slip rate may reflect a component of aseismic deformation resulting from localized diapirism or dissolution-induced subsidence associated with subsurface evaporite beds in the Middle Jurassic Arapien Shale.

Structurally, the Levan-Fayette segment boundary appears to be a relay ramp, and includes fault scarps and lineaments that accommodate a left-stepping transfer of displacement between the two segments.